This invention relates to a device and method for testing the mesopic visual acuity of a subject and more particularly to an automated device for giving the dark adaptation vision characteristics of a subject with respect to time.
It is well known that in the nearly complete absence of light, a person's vision is "scotopic," that is, light is perceived primarily by the rods in the eye. In normal daylight vision a person's vision is primarily "phototopic;" the cone vision controls. Intermediate between scotopic and phototopic vision is the range of mesopic vision. If the illumination is suddenly decreased from phototopic levels to mesopic levels, the visual acuity of human beings falls regularly and predictably and then increases with respect to time for a given person, as his eyes adapt to the lower level of illumination. This acuity characteristic is different for each person and can deteriorate with age or illness.
In some prior art visual acuity devices, a person's visual acuity at only a single light level is determined. In still another device, the time required for the subject's vision to adapt to two predetermined light intensities is measured. The problem with these types of devices is that they either only measure a threshold light perception level at some fixed point in time or they measure the time required for the person's eye to adapt to one or more predetermined light intensities. They do not, however, give a complete time based characteristic of the light intensity levels which may be perceived by the subject during a single course of dark adaptation.
Still another problem with many such prior art devices is that they either must be conducted in a darkened room or they require elaborate light excluding shields which are attached to the optical measuring device, thereby making it either uncomfortable for the subject and the observer to use and/or bulky. Since it is sometimes necessary for the subject to look into the testing device for periods of time ranging as long as 15 minutes to half an hour, it is desirable that such devices be comfortable for the subject.